1. Field of the Invention
This invention relates to vertical-cavity surface-emitting lasers (VCSELs) and, in particular, to optically-pumped multiple-quantum well (MQW) active regions for devices such as OP VCSELs.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, free electron lasers, and semiconductor lasers. All lasers have a laser cavity defined by at least two laser cavity mirrors, and an optical gain medium in the laser cavity. The gain medium amplifies electromagnetic waves (light) in the cavity by stimulated emission, thereby providing optical gain.
In semiconductor lasers, a semiconductor active region serves as the optical gain medium. Semiconductor lasers may be diode (bipolar) lasers or non-diode, unipolar lasers such as quantum cascade (QC) lasers. Semiconductor lasers are used for a variety of industrial and scientific applications and can be built with a variety of structures and semiconductor materials.
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
The optical gain of a laser is a measure of how well a gain medium such as an active region amplifies photons by stimulated emission. The primary function of the active region in a semiconductor laser is to provide sufficient laser gain to permit lasing to occur. The active region may employ various materials and structures to provide a suitable collection of atoms or molecules capable of undergoing stimulated emission at a given lasing wavelength, so as to amplify light at this wavelength. The active region may comprise, for example, a superlattice structure, or a single- or multiple-quantum well (MQW) structure.
Amplification by stimulated emission in the active region of a semiconductor laser is described as follows. The semiconductor active region contains some electrons at a higher, excited state or energy level, and some at a lower, resting (ground) state or energy level. The number and percentage of excited electrons can be increased by pumping the active region with a pumping energy, from some energy source such as an electrical current or optical pump. Excited electrons spontaneously fall to a lower state, “recombining” with a hole. The recombination may be either radiative or non-radiative. When radiative recombination occurs, a photon is emitted with the same energy as the difference in energy between the hole and electron energy states.
Stimulated emission, as opposed to spontaneous emission, occurs when radiative recombination of an electron-hole pair is stimulated by interaction with a photon. In particular, stimulated emission occurs when a photon with an energy equal to the difference between an electron's energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a second photon. The second photon has the unique property that it has the same energy, frequency, and phase as the original photon. Thus, when the photons produced by spontaneous (or stimulated) emission interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. (Viewed as waves, the atom emits a wave having twice the amplitude as that of the original photon interacting with the atom.) I.e., one photon of a given energy, frequency, and phase produces a second photon of the same energy, frequency, and phase; and these two photons may each, if not absorbed, stimulate further photon emissions, some of which can themselves stimulate further emissions, and so on.
Amplification by stimulated emission requires that more photons be produced by stimulated emission than are absorbed by lower-state electrons. This condition, known as population inversion, occurs when there are more excited (upper lasing level) electrons than ground-state (lower lasing level) electrons. If there were more lower state than upper state electrons, then more photons would be absorbed by the lower energy electrons (causing upward excitations) than would be produced by stimulated emission. When there is a population inversion, however, enough electrons are in the excited state so as to prevent absorption by ground-state electrons from sabotaging the amplification process. Thus, when population inversion is achieved, stimulated emission predominates over stimulated absorption, thus producing amplication of light (optical gain). If there is population inversion, lasing is therefore possible, if other necessary conditions are also present.
Population inversion is achieved by applying a sufficient pumping energy to the active region, to raise a sufficient number of electrons to the excited state. Various forms of pumping energy may be utilized to excite electrons in the active region and to achieve population inversion and lasing. For example, semiconductor lasers of various types may be electrically pumped (EP), by a DC or alternating current. Optical pumping (OP) or other pumping methods, such as electron beam pumping, may also be used. EP semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. As a result of the potential applied, charge carriers (electrons and holes) are injected from opposite directions into an active region. This gives rise to an increase in spontaneous generation of photons, and also increases the number of excited state electrons so as to achieve population inversion. Thus, in electrical pumping, carriers injected across electrical semiconductor junctions recombine in active layers and thereby generate laser radiation.
In OP lasers, an external laser beam or other light is directed into the active region, where the light is absorbed, thus generating carriers, some of which recombine to emit radiation at the desired wavelength in the quantum wells.
In a semiconductor laser, an active region is sandwiched between the cavity mirrors, and pumped with a pumping energy to cause population inversion. Photons are spontaneously emitted in the active region. Some of those photons travel in a direction perpendicular to the reflectors of the laser cavity. As a result of the ensuing reflections, the photons travel through the active region multiple times, being amplified by stimulated emission on each pass through the active region. Thus, photons reflecting in the cavity experience gain when they pass through the active region. However, loss is also experienced in the cavity, for example by extraction of the output laser beam, which can be about 1% of the coherent cavity light, by absorption or scattering caused by less than perfect (100%) reflectance (reflectivity) of the cavity mirrors, and other causes of loss.
Therefore, for lasing to occur, there must be not only gain (amplification by stimulated emission) in the active region, but enough gain to overcome all losses in the laser cavity as well as allow an output beam to be extracted, while still allowing laser action to continue. The minimum gain provided the active region that will permit lasing, given the cavity losses, is the threshold lasing gain of the laser medium.
The gain of a semiconductor wavelength varies depending on the wavelength of light. When the active region provides the threshold lasing gain over a given wavelength range, there will be a sufficient amount of radiative recombinations stimulated by photons, so that the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and lasing. This causes coherent light to build up in the resonant cavity formed by the two mirrors, a portion of which passes through one of the mirrors (the “exit” mirror) as the output laser beam.
Because a coherent beam makes multiple passes through the optical cavity, an interference-induced longitudinal mode structure or wave is observed. The wave along the laser cavity is a standing EM wave and the cavity of effective optical length L only resonates when the effective optical path difference between the reflected wavefronts is an integral number of whole wavelengths (the effective cavity length or optical path difference takes phase-shifting effects at the mirrors into account). In other words, lasing is only possible at wavelengths for which the round-trip phase is a multiple of 2π. The set of possible wavelengths that satisfy the standing wave condition is termed the set of longitudinal modes of the cavity. Although there are an infinite number of such wavelengths, only a finite number of these fall within the wavelength range over which the gain spectrum of the active region exceeds the threshold lasing gain. The laser will lase only at one or more of the possible longitudinal (wavelength) modes which fit into this wavelength range.
Semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, in contrast to SELs, in which the radiation output is perpendicular to the wafer surface, as the name implies. The most common type of SEL is the vertical-cavity surface-emitting laser (VCSEL). Both EP and OP VCSEL designs are possible. The “vertical” direction in a VCSEL is the direction perpendicular to the plane of the substrate on which the constituent layers are deposited or epitaxially grown, with “up” being typically defined as the direction of epitaxial growth. In some designs, the output laser beam is emitted out of the top side, in which case the top mirror is the exit mirror. In other designs, the laser beam is emitted from the bottom side, in which case the bottom mirror is the exit mirror. The exit mirror typically has slightly lower reflectivity than the other (“backside”) mirror.
VCSELs have many attractive features compared to edge-emitting lasers, such as low threshold current, single longitudinal mode, a circular output beam profile, scalability to monolithic laser arrays, and ease of fiber coupling. The shorter cavity resonator of the VCSEL provides for better longitudinal mode selectivity, and hence narrower linewidths. Because of their short cavity lengths, VCSELs have inherent single-frequency operation. Additionally, because the output is perpendicular to the wafer surface, it is possible to test fabricated VCSELs on the wafer before extensive packaging is done (wafer scale probing), in contrast to edge-emitting lasers, which must be cut from the wafer to test the laser. Also, because the cavity resonator of the VCSEL is perpendicular to the layers, there is no need for the cleaving operation common to edge-emitting lasers.
The VCSEL structure usually consists of an active (optical gain) region or layer sandwiched between two mirrors, such as distributed Bragg reflector (DBR) mirrors. DBRs are typically formed of multiple pairs of layers referred to as mirror pairs; DBRs are sometimes referred to as mirror stacks. The DBR mirrors of a typical VCSEL can be constructed from dielectric (insulating) or semiconductor layers (or a combination of both, including metal mirror sections). The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction, i.e. the DBR comprises alternating layers of high and low indexes of refraction. For semiconductor DBRs, the layers are typically selected so that they are easily lattice matched to the other portions of the VCSEL, to permit epitaxial fabrication thereof.
The two mirrors may be referred to as a top DBR and a bottom DBR; the top DBR often serves as the exit mirror. Because the optical gain is low in a vertical cavity design compared to an edge-emitting laser (because the photons in the cavity pass through the active region for a smaller percentage of the round-trip optical path), the reflectors require a high reflectivity in order to achieve a sufficient level of feedback for the device to lase.
For semiconductor DBRs, the number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectivity). The difference between the refractive indices of the layers of the mirror pairs can be higher in dielectric DBRs, generally imparting higher reflectivity to dielectric DBRs than to semiconductor DBRs for the same number of mirror pairs and DBR thickness. Conversely, in a dielectric DBR, a smaller number of mirror pairs can achieve the same reflectivity as a larger number in a semiconductor DBR. However, it is sometimes necessary or desirable to use semiconductor DBRs, despite their lower reflectivity/greater thickness, to conduct current, for example (e.g., in an EP VCSEL). Semiconductor DBRs also have higher thermal (heat) conductivity than do dielectric DBRs, making them more desirable for heat-removal purposes, other things being equal. Semiconductor DBRs may also be preferred for manufacturing reasons (e.g., a thicker DBR may be needed for support) or fabrication reasons (e.g., an epitaxial, i.e. semiconductor, DBR may be needed if other epitaxial layers need to be grown on top of the DBR).
When properly designed, these mirror pairs will cause a desired reflectivity at the laser wavelength. VCSEL mirrors are typically designed so that the bottom (backside) DBR mirror (i.e. the one interposed between the substrate material and the active region) has nearly 100% reflectivity, while the top (exit) DBR mirror has a reflectivity that may be 98%-99.5% (depending on the details of the laser design). The partially reflective top mirror passes a portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors. Of course, as noted above, in other designs, the bottom mirror may serve as the exit mirror, with the top mirror having the higher reflectivity.
OP VCSELs are typically pumped by a high-power edge-emitting diode pump laser. This makes it possible to achieve high single-transverse-mode output power for both long and short-wavelength lasers. Also, unlike EP VCSELs, OP VCSELs do not need electrical contacts, doping of the semiconductor material, or current confinement structures, which can make manufacturing simpler and less costly. Additionally, since no current needs to pass through the DBRs, they can be undoped which reduces the optical losses in the lasing cavity, compared to EP VCSEL designs having doped DBR and other epi layers. Moreover, self-heating is reduced by optically pumping to inject carriers directly into the active region, resulting in increased output power and operating temperature.
The “top” mirror of an OP VCSEL may be a DBR mirror integrated with the laser structure, or mounted external to the monolithic structure, forming an external cavity. VCSELs employing external cavity mirrors are sometimes referred to as vertical external-cavity surface-emitting lasers (VECSELs), but are referred to herein as VCSELs for simplicity. Such an external cavity can be used for frequency doubling or absorption spectroscopy, for example.
The active region of OP VCSELs typically employs an MQW structure having a plurality of quantum wells (QWs) having separator layers between successive ones thereof. The QW layers are typically very thin, e.g. on the order of 100-150 Å, and are spaced apart by separator layers composed of a semiconductor material having a higher conducting band energy than that of the QW layers. The separator layers are also referred to as absorbing layers, because they absorb the pump radiation provided by the pump laser. The absorbing layers have a higher conduction band energy that the QW layers. The active or QW layers and separator/absorbing layers together constitute the gain structure of an OP laser.
In an OP VCSEL, pump energy in the form of light (radiation) is typically directed into the active region, typically through a “top” side of the active region layers, where it is absorbed by the separator layers, thereby raising existing electrons to higher energy states or levels. (For a defined cavity, the pump light is directed into the cavity; where a wafer having no mesas or other definition of cavities is provided, the direction of pump light into a location of a wafer generates and defines a cavity.) Raising the existing electrons to higher energy states generates electrical carriers (holes and electrons), which relax or “fall” into neighboring quantum wells where they are “trapped,” thus creating a large concentration of electrical carriers in the QW layers. Carrier recombination in the QW lawyers generates electromagnetic radiation at the fundamental wavelength. The QW layers are typically arranged so that they are spaced apart by one half-wavelength of the fundamental lasing wavelength, and so that they correspond in position with antinodes of the standing wave of the fundamental laser radiation that exists in the laser cavity when lasing is occuring.
The pumping light passes through successive separator layers, each of which absorbs some of the light passing through it. Therefore, the quantum wells and separator layers closer to the light source (e.g., those at the top of the active layer) absorb a relatively larger percentage of the light, and thus have a relatively larger amount of recombinations, than those farther away (e.g., near the bottom of the active region). For example, for an active region having 4 QWs, each at an antinode, the first may receive carriers due to absorption of about 40-50% of the optical pumping light, with each succeeding QW having diminishing amounts of recombination due to the declining percentage of light remaining to be absorbed in the vicinity of each QW. Such a nonuniform distribution of optical pumping power leads to lower maximum output power and/or inefficient use of some of the QWs, e.g. those having comparatively smaller amounts of carrier recombination.
There is, therefore, a need for improved OP VCSEL and MQW active region techniques and structures having more uniform absorption of pumping power.